1. Field of the Invention
The present invention relates to an ink composition, and more particularly, to an ink composition comprising an amide compound which has a low mobility of a colorant and a high adhesion to paper, thus, minimizing bleeding of colors of printed images.
2. Description of the Related Art
In general, colorants are substances which exhibit inherent colors by selectively absorbing and reflecting visible light. Colorants are classified into dyes and pigments.
Dyes, which are soluble in solvents, are absorbed into any materials to be dyed, such as fibers, leathers, furs, papers, and the like, thus providing considerable fastness to daylight, washing, friction, and the like. Pigments, which are insoluble in solvents, are coloring matters in the form of particulates, and are not absorbed into the material to be dyed, but are adhered to the surface of the material to be dyed by physical connection (e.g., adhesion, and the like.), thus providing their inherent colors.
Since pigments are generally insoluble in the solvents, a uniform dispersion of pigment particulates in a solution is vital to continuously maintain a stable dispersed state.
A water soluble, dye-type ink has effective long-term storage stability, its uniformity is maintained, and its color is clear and brightness is high, while it is weak in water fastness, light resistance, and the like.
A pigment-type ink has a high optical density (OD), and effective water fastness and light resistance, and less bleeding between colors, while its color clearness is poor and its long-term storage stability is weak compared to the dye-type ink. Also, the images printed by the pigment-type inks have weak dry and wet rubfastnesses. When color printing (multicolor printing) with dyes or pigments, bleeding at interfaces of each color occurs, and thus clearness of images is weakened.
To improve rubfastness and color fastness by decreasing mobility of a colorant on a substrate, an ink composition for ink-jets comprising a pigment and a water-soluble resin (U.S. Pat. No. 5,172,133) and an ink composition comprising a specific alcohol as an additive (U.S. Pat. No. 5,529,616) are disclosed.
However, the ink-jet ink is used in jet printing through nozzles, and thus its low viscosity should be maintained. An amount of a polymer binder used in the ink is limited. The binder used in a limited amount cannot bind pigment particles to the substrate sufficiently, thus providing unsatisfactory rubfastness and color fastness.
As another approach to improve rubfastness and color fastness by decreasing mobility of a coloring agent on a substrate, a method including chelating a metal ion (U.S. Pat. No. 4,694,302), a method including reacting a cation with an anion (U.S. Pat. No. 5,623,294), and a method by reaction of a polymer (U.S. Pat. No. 5, 629,359) are disclosed.
However, the method including chelating a metal ion has difficulty in obtaining long-term storage stability due to a reaction of each ink constituent with a metal ion contained in ink. The method including reacting a cation with an anion also has ineffective performance due not only to a reaction between ink constituents, but also a reaction of a substrate and ion constituents. Also, the method by reaction of a polymer is environmentally harmful due to an unreacted monomer and needs a curing apparatus and curing time, in addition to the difficulty in obtaining long-term storage stability.
Methods of improving storage stability and wetness of an ink composition before jetting are disclosed (U.S. Pat. Nos. 5,108,503 and 5,981,623). In these methods, various heterocyclic compounds containing nitrogen, such as 2-pyrrolidone, N-methyl-2-pyrrolidone, and 1,3-dimethyl-2-imidazolidinone, are used as additives to improve the performance of the ink composition.